Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

[James Bottomley]

On Tue, 2017-02-07 at 11:49 -0800, Christoph Hellwig wrote:
> On Tue, Feb 07, 2017 at 11:02:03AM -0800, James Bottomley wrote:
> > >   Another option would be to require something like a project as
> > > used 
> > > for project quotas as the root.  This would also be conveniant as
> > > it 
> > > could storge the used remapping tables.
> > 
> > So this would be like the current project quota except set on a
> > subtree?  I could see it being done that way but I don't see what
> > advantage it has over using flags in the subtree itself (the
> > mapping is
> > known based on the mount namespace, so there's really only a single
> > bit
> > of information to store).
> 
> projects (which are the underling concept for project quotas) are
> per-subtree in practice - the flag is set on an inode and then
> all directories and files underneath inherit the project ID,
> hardlinking outside a project is prohinited.

OK, this is what I don't understand: how is something that's inode
based limited to be per-subtree?  The way I've seen the VFS operate it
seems that any given inode (and indeed dentry) can appear in many
subtrees so how do I limit them to just one?

James